1. Field of the Invention
The invention relates to a mechanism for driving a movable body used in a reproducing apparatus, more particularly to a mechanism for driving an optical system or an original platform which move reciprocatingly in a reproducing apparatus.
2. Description of the Prior Art
In a conventional reproducing apparatus, a movable body making a reciprocating movement is usually connected with a driving source by a conduction mechanism wound therearound.
In FIG. 1, there is shown a schematic drawing illustrating a conventional reproducing apparatus wherein an original is fixed and an optical system is movable. In the schematic drawing, reference number 1 denotes a fixed original platform on which an original (not shown) is laid. Under the original platform 1 are supported a first movable mirror 2 and a second movable mirror 3 upon a support (not shown) in such a way that they may be movable parallel to the original platform 1 and movably mounted to guide members.
First movable mirror 2 and second movable mirror 3 reciprocatingly move in the direction of the length of original platform 1 under the principle of a moving pulley which will be explained in detail later. On both sides of its moving region, there are provided a pair of fixed pulleys 4 and 5 respectively. Further, upon the support (not shown) for the second movable mirror 3, there is provided a double slotted moving pulley 6.
Between the fixed pulleys 4 and 5, there is provided a double slotted guide pulley 7 and under the guide pulley 7, there is provided a driving pulley 8. A wire 9 one end 9a of which is fixed to an immovable member 10 is entrained over a moving pulley 6 in the form of U-shaped figure and after being turned back thereat, the wire is entrained over a fixed pulley 4 on one side and is afterwards turned back again thereat, the wire is guided to a driving pulley 8 via a guide pulley 7. Wire 9 is wound upon the driving pulley 8 in a plurality of turns and after being entrained over guide pulley 7, is fixed to the first movable mirror 2 after being turned back in another fixed pulley 5, and is turned back in the form of U-shaped figure after being entrained again on the moving pulley 6.
The other end 9b of the wire 9 wound upon each pulley, as mentioned above and to which the first movable mirror 2 is fixed, is fixed to an immovable member 10.
When the driving pulley 8 being rotated by a driving source (not shown) in the advancing direction shown by an arrow a, the first movable mirror 2 connected with the driving pulley 8 via the wire 9 moves at the speed V in the returning direction shown by an arrow a1. At this time, as the moving pulley 6 mounted upon the second movable mirror 3 moves under the principle of a moving pulley, the second movable mirror 3 makes a follow movement at the speed 1/2V in the direction shown by an arrow a2.
Further, the first and second movable mirrors 2 and 3 shown in the FIG. 1 are in positions which are departed to some extent from their home or original positions.
The advancing rotation of the driving pulley 8 lasts until first movable mirror 2 has moved from the left end of the original platform 1 to the right end. At this time, the first movable mirror 2 scans an original image illuminated by a light source (not shown) in a slit form and its light image is reflected toward the second movable mirror 3. The light image reflected by the second movable mirror 3 is guided to an exposure station of a reproducing apparatus (not shown).
When the first movable mirror 2 and second movable mirror 3 move to the right end of the original platform 1, the driving pulley 8 begins to rotate in the direction of returning shown in the drawing by an arrow b. Then, the first and second movable mirrors 2 and 3 move in the returning direction shown by arrows b1 and b2 respectively and return to their original positions.
Wire 9 connecting the driving pulley 8 as a driving member and the first and second movable mirrors 2 and 3 as movable bodies is wound with some slack for enabling position adjustment of each movable mirror, for instance. Further, if the first and second movable mirrors 2 and 3 were not moved at a predetermined constant speed, the formation of a light image in an exposure station would be disturbed.
Therefore, the above mentioned wire 9 is provided with a tension mechanism for absorbing the above mentioned slack. This tension mechanism is usually provided in the slack portion of the wire. In the apparatus shown in FIG. 1, a tension pulley 11 is also provided on the wire portion 9c between a guide pulley 7 and the driving pulley 8 which becomes a slack portion at the time of advancing movement of a movable body.
The tension pulley 11 is forced to move in the direction shown by an arrow owing to a tension spring (not shown) and absorbs the slack of wire induced during the advancing movement. Therefore, the advancing movement of first and second movable mirrors 2 and 3 is stabilized by the provision of a tension pulley 11.
However, when a movable body makes advancing and returning movements, the tension side and the slack side of the wire 9 will be reversed during the advancing movement and returning movement. Therefore, in FIG. 1, the wire portion 9c between the guide pulley 7 and the driving pulley 8 which was a slack portion during the advancing movement will be changed into a tension portion during the time of the returning movement.
During the returning movement in which the driving pulley 8 rotates in the direction shown by an arrow b, the tension pulley 11 will be moved against the elasticity of a tension spring (not shown) as shown by a dotted line 11A owing to the fact that the wire portion 9c entrained over the tension pulley 11 has become a tension portion (or side).
The tension pulley 11 during the returning movement is desired to stop at its station as shown by a dotted line 11A, even temporarily. However, owing to the impact of the wire induced at the time of exchanging from advancing movement to returning movement, a tension spring (not shown) causes vibrating movement. Thus, during the returning movement of a movable body, the vibration phenomenon of the tension pulley between the real line position and the dotted line position will be induced. This vibration of the tension pulley will be conducted to the movable body as an extension or contraction of the wire.
The first and second movable mirrors 2 and 3 will vibrate before and behind the movable body in the direction of movement during the returning movement. In other words, a so-called boundary will be induced and the movable mirrors 2 and 3 thereby can not move at a constant speed. Therefore, the impact at the time when the movable mirrors have returned to their original positions will become large and an impact sound will be uttered and starting positions at their home positions will be undesirably changed. Further, owing to the shock, illumination lamps (not shown) which are provided substantially in a body with the first movable mirror 2 will be broken or the wire will likely be displaced from the pulleys.
Especially in a high speed reproducing apparatus, the advancing and returning speed of a movable body is usually raised. In some cases, the speed of a movable body during the returning movement is raised to 2-3 that of the speed during the advancing movement. Accordingly, the above mentioned shock of a movable body will become large and the starting positions at the time of repeated copying will not become constant.
The reasons for the induction of the boundary of a movable body are that the tension side and the slack side of a wire will be changed during the advancing and returning movements and that the tension mechanism can not absorb the slack in the wire thereby induced.
The most simple procedure for preventing the boundary of a movable body is to replace a tension spring with a strong tension spring and to make the position of a tension pulley during the returning movement unchangeable. However, by doing so, the wire will be loaded under large tension during the advancing movement of the movable body. The load to the wire, pulley or driving source will also be greatly increased. More especially, a problem in design to the effect that excess loads will be transmitted to a bearing will appear.